Locus equations were employed to phonetically describe stop place categories as a function of syllable-initial, -medial, and -final position. Ten speakers, five male and five female, produced a total of 2700 CVC and 4500 VCV utterances that were acoustically analyzed to obtain F2 onset, F2 vowel, and F2 offset frequencies for locus equation regression analyses. In general, degree of coarticulation, as indexed by locus equation slope, was reduced for post-vocalic (VC) stops relative to pre-vocalic stops (pooled data from initial and medial positions), but significant differences were observed as a function of stop consonant. All stops showed significantly reduced R2 values and increased standard errors of estimate for VC relative to CV productions. Separability of stop place categories in a higher-order slope X y-intercept acoustic space also diminished for VC vs CV stop productions. The degradation of classic locus equation form (high correlation and linearity) for VC relative to CV productions was attributed to greater articulatory precision in the production of pre-vocalic compared to post-vocalic stops. This greater articulatory precision was interpreted as reflecting a greater need to normalize vowel context-induced variability of the F2 transition for syllable onset relative to final stops. The decline in acoustic lawfulness of syllable-final stops is discussed in terms of coarticulatory interactions and expected perceptual correlates.

Across much of the United States, the phonemic contrast between the vowels of cot and caught is being lost through a sound change known as the low back vowel merger. This paper examines the spread of this merger in the state of Missouri. Acoustic examination of F0, F1,F2, F3, and vowel duration reveals that speakers in the greater St. Louis area maintain the phonemic distinction between /a/ andɔ/, while in much of the state, this distinction is being lost or diminished. In addition to static formant measures, the formant trajectories of the two vowels are examined, and it is found that in St. Louis speech, the VC consonantal transition of F2 is accomplished more quickly for/ɔ/ than for /a/. Although the F2 transitions of the two vowels differ, their overall spectral shapes are more similar than other comparable vowel pairs such as /æ/ and /g=e/, which are not undergoing widespread merger. The dynamic similarity between /a/ and /ɔ/ is posited as a partial explanation for why this particular merger is spreading so rapidly throughout the United States.